A method includes the following steps: a first shell (12) is immobilized in a first shell carrier (6); a second shell (29) is immobilized in a second shell carrier (27); the first shell carrier (6) is oriented in such a way that a useful moulding surface of the first shell (12) faces upwards and forms the base of a vessel; a pre-determined quantity of polymerizable material is placed in the vessel; the first shell carrier (6) and the second shell carrier (27) are moved into a pre-determined relative position wherein the first shell (12) and the second shell (29) are in a pre-determined moulding position, by at least partially arranging the second shell (29) in the vessel, at a distance from the edges thereof. The inventive device includes a structure for positioning the shell carriers.